Method for dynamic CSI feedback

ABSTRACT

Systems and methods for providing Channel State Information (CSI) feedback in a cellular communications network are disclosed. In some embodiments, a base station of a cellular communications network disables inter-subframe channel interpolation of CSI-RS estimates across subframes at the wireless device and receives one or more CSI reports from the wireless device that are generated by the wireless device with inter-subframe channel interpolation of CSI-RS estimates across subframes disabled in response to the base station disabling inter-subframe channel interpolation of CSI-RS estimates across subframes at the wireless device. In this manner, CSI feedback is improved particularly in embodiments in which the base station transmits a beamformed CSI-RS resource(s) and reuses the same CSI-RS resource(s) for different beams over time.

This application is a 35 U.S.C. § 371 national phase filing ofInternational Application No. PCT/SE2015/051056, filed Oct. 6, 2015,which claims priority to U.S. Provisional Application No. 62/062,397,filed Oct. 10, 2014, the disclosures of which are incorporated herein byreference in their entireties.

TECHNICAL FIELD

The present disclosure relates to Channel State Information (CSI)feedback in a cellular communications network.

BACKGROUND

Long-Term Evolution (LTE) uses Orthogonal Frequency DivisionMultiplexing (OFDM) in the downlink and Discrete Fourier Transform(DFT)-spread OFDM in the uplink. The basic LTE downlink physicalresource can thus be seen as a time-frequency grid as illustrated inFIG. 1, where each resource element corresponds to one OFDM subcarrierduring one OFDM symbol interval.

As illustrated in FIG. 2, in the time domain, LTE downlink transmissionsare organized into radio frames of 10 milliseconds (ms), each radioframe consisting of ten equally-sized subframes of length T_(SUBFRAME)=1ms. For normal cyclic prefix, one subframe consists of 14 OFDM symbols.The duration of each OFDM symbol is approximately 71.4 microseconds(μs).

Furthermore, the resource allocation in LTE is typically described interms of Resource Blocks (RBs), where a RB corresponds to one slot (0.5ms) in the time domain and 12 contiguous subcarriers in the frequencydomain. A pair of two adjacent RBs in the time direction (1.0 ms) isknown as a RB pair. RBs are numbered in the frequency domain startingwith 0 from one end of the system bandwidth.

Downlink transmissions are dynamically scheduled. In particular, in eachsubframe, the base station transmits control information about theterminals (i.e., User Equipment devices (UEs)) to which data istransmitted in the current downlink subframe. This control signaling,which is carried over the Physical Downlink Control Channel (PDCCH), istypically transmitted in the first 1, 2, 3, or 4 OFDM symbols in eachsubframe, where the number n=1, 2, 3, or 4 is known as the ControlFormat Indicator (CFI). The downlink subframe also contains commonreference symbols, which are known to the receiver and used for coherentdemodulation of, e.g., the control information. A downlink system withCFI=3 OFDM symbols as control is illustrated in FIG. 3.

From LTE Release 11 onwards, the above-described resource assignmentscan also be scheduled on the Enhanced Physical Downlink Control Channel(EPDCCH). For Release 8 to Release 10 only the PDCCH is available.

The reference symbols shown in FIG. 3 are the Cell-Specific ReferenceSymbols (CRSs). The CRSs are used to support multiple functionsincluding fine time- and frequency-synchronization and channelestimation for certain transmission modes.

In a cellular communications system, there is a need to measure thechannel conditions in order to know what transmission parameters to use.These parameters include, e.g., modulation type, coding rate,transmission rank, and frequency allocation. This applies to Uplink (UL)as well as Downlink (DL) transmissions.

The scheduler that makes the decisions on the transmission parameters istypically located in the base station (i.e., the enhanced or evolvedNode B (eNB)). Hence, the scheduler can measure channel properties ofthe UL directly using known reference signals that the terminals (i.e.,UEs) transmit. These measurements then form a basis for the ULscheduling decisions that the eNB makes, which are then sent to the UEsvia a DL control channel. Conversely, for the DL, the scheduler receivesChannel State Information (CSI) feedback from the terminals, which istaken into consideration by the scheduler when selecting thetransmission parameters for the DL transmissions to those terminals.

In LTE Release 8, CRSs are used in the DL for CSI estimation andfeedback, and for channel estimation for demodulation. CRSs aretransmitted in every subframe and are defined to support up to fourAntenna Ports (APs). In LTE Release 10, to support up to eight APs, CSIReference Signals (CSI-RSs) are defined for the UE to measure and feedback CSI relating to the multiple APs. Each CSI-RS resource consists oftwo Resource Elements (REs) over two consecutive OFDM symbols, and twodifferent CSI-RSs (for two different APs) can share the same CSI-RSresource (two REs) by Code-Division Multiplexing (CDM). Also, a CSI-RScan be transmitted once per 5, 10, 20, 40, or 80 ms, where this timingis referred to as the CSI-RS periodicity. Therefore. CSI-RS has loweroverhead and lower duty-cycle as compared to CRS. On the other hand,unlike CRS, CSI-RS is not used as a demodulation reference. DifferentCSI-RSs can also be transmitted with different offsets in the subframe,where the offset of the CSI-RS within the subframe is referred to as theCSI-RS subframe offset. When a CSI-RS is configured, the UE measures thechannel for a given AP at each time instant and may interpolate thechannel in between CSI-RS occasions to estimate the dynamically varyingchannel, e.g., by one interpolated sample per 1 ms instead of, e.g., onemeasured sample each 5 ms.

FIGS. 4A and 4B show examples of mappings from different CSI-RSconfigurations to REs in a RB pair. FIG. 4A illustrates the mapping forone or two APs, where 20 configurations are possible. The two CSI-RSs ofthe two APs of a particular cell can be transmitted by, for instance,configuration 0 by CDM, while CSI-RSs of APs of other neighboring cellscan be transmitted by configuration j, with 1<=j<=19, to avoid referencesignal collisions with the CSI-RS in the cell. FIG. 4B shows the mappingfor four APs, where 10 configurations are possible. The four CSI-RSs ofthe four APs of a particular cell can be transmitted by, for instance,configuration 0 by CDM, while CSI-RSs of APs of other neighboring cellscan be transmitted by configuration j, with 1<=j<=9.

The OFDM symbols used by the two consecutive REs for one CSI-RS areQuadrature Phase Shift Keying (QPSK) symbols, which are derived from aspecified pseudo-random sequence. To randomize the interference, theinitial state of the pseudo-random sequence generator is determined bythe detected cell Identifier (ID) or a virtual cell ID configured to theUE by Radio Resource Control (RRC) signaling. CSI-RS with suchnon-zero-power OFDM symbols are called Non-Zero-Power (NZP) CSI-RS.

On the other hand, Zero-Power (ZP) CSI-RS can also be RRC-configured tothe UE for the purpose of Interference Measurement (IM) (in TransmissionMode 10 (TM10) only), or for the purpose of improving the CSI estimationin other cells (in Transmission Mode 9 (TM9) or TM10). However, theCSI-RS mapping with four APs will always be used by the ZP CSI-RS. Forexample, in FIG. 4B, if configuration 0 with NZP CSI-RS is used by cellA to estimate the CSI of the two APs in cell A, configuration 0 with ZPCSI-RS (a total of four REs per RB pair) can be used by the neighboringcell B to minimize the DL interference to cell A over the four REs inconfiguration 0, such that the CSI estimation of the two APs in cell Acan be improved.

In LTE TM10, up to four CSI processes and three NZP CSI-RS can beconfigured for a UE by RRC signaling. These four CSI processes can, forinstance, be used to acquire CSI for APs in up to three different cells(or Transmission Points (TPs) within the same cell) in a CoordinatedMultipoint (CoMP) framework. The four CSI processes can also be assignedto multiple different beams transmitted from the same eNB using an arrayantenna that is capable of beamforming in azimuth, elevation, or both(i.e., Two-Dimensional (2D) beamforming). See 3^(rd) GenerationPartnership Project (3GPP) Technical Specification (TS) 36.213 V12.3.0,3GPP TS 36.331 V12.3.0, and 3GPP TS 36.211 V12.3.0 for complete LTEspecifications on how CSI processes and CSI-RS configurations are setup. A beam of a transmitted signal, such as a CSI-RS, is obtained bytransmitting the same signal from multiple antenna elements in an array,but with individually controlled phase shifts (and potentially amplitudetapering) for each antenna element. The resulting radiation pattern ofthe transmitted signal thus has a different beam width and main pointingdirection compared to the antenna element radiation pattern. Hence, abeamformed signal, such as beamformed CSI-RS, is obtained. Typically,the antenna elements at the transmitter are closely spaced, as toachieve correlated channels, which makes the beamforming more effective.The benefits of beamforming is reduced interference (due to thetypically narrow beam width of the transmitted signal) and increasedeffective channel gain (due to the applied beamforming phase shifts atthe transmitter which ensure a coherent addition of the signals fromeach transmit antenna at the receiver).

In order for the UE to derive the correct CSI, each CSI process in TM10is associated (and configured by RRC signaling) with a signal hypothesisand an interference hypothesis. The signal hypothesis describes whichNZP CSI-RS reflects the desired signal. The interference is measured ina configured CSI-IM resource, which is similar to a CSI-RS with four REsper Physical Resource Block (PRB) pair, which the UE uses forinterference measurements. To better support the IM in CoMP, CSI-IM isstandardized and is based on the ZP CSI-RS. Therefore, each of the up tofour CSI processes consists of one NZP CSI-RS and one CSI-IM.

For a TM9 UE, only a single CSI process can be configured, and no CSI-IMis defined. The IM is thus unspecified in TM9. There is however still apossibility to get CSI feedback from two different Subframe (SF) sets:SF set 1 and SF set 2. For instance, based on, e.g., the Almost BlankSubframe (ABS) information signaled over X2, a pico eNB can configure aUE to feed back CSI for both protected (i.e., Reduced Power Subframes(RPSF)) subframes (where a corresponding macro eNB has reduced activity)and CSI for unprotected subframes in two different CSI reports. Thisgives the pico eNB information to perform link adaptation in the twotypes of subframes differently, depending on whether it is a protectedsubframe or not. It is also possible for a UE configured in TM10 to useboth subframe sets and multiple CSI processes.

In LTE, the format of the CSI reports are specified in detail and maycontain Channel Quality Information (CQI), Rank Indicator (RI), andPrecoding Matrix Indicator (PMI). See 3GPP TS 36.213 V12.3.0. Thereports can be wideband or applicable to subbands. They can beconfigured by a RRC message to be sent periodically or in an aperiodicmanner or triggered by a control message from the eNB to a UE. Thequality and reliability of the CSI are crucial for the eNB in order tomake the best possible scheduling decisions for the upcoming DLtransmissions.

The LTE standard does not specify how the UE should obtain and averagethe CSI-RS and CSI-IM measurements from multiple time instants, i.e.,subframes. For example, the UE may measure over a time frame of multiplesubframes, unknown to the eNB and combine several measurements in aUE-proprietary way to create the CSI values that are reported, eitherperiodically or triggered.

In the context of LTE, the resources (i.e., the REs) available fortransmission of CSI-RS are referred to as “CSI-RS resources,” Inaddition, there are also “CSI-IM resources.” The latter are defined fromthe same set of possible physical locations in the time/frequency gridas the CSI-RS, but with zero power, hence ZP CSI-RS. In other words,they are “silent” CSI-RSs and when the eNB is transmitting the shareddata channel, it avoids mapping data to those REs used for CSI-IM. Theseare intended to give a UE the possibility to measure the power of anyinterference from another transmitter other than the serving node of theUE.

Each UE can be configured with one, three, or four different CSIprocesses. Each CSI process is associated with one CSI-RS and one CSI-IMresource where these CSI-RS resources have been configured to the UE byRRC signaling and are thus periodically transmitted/occurring with aperiodicity of T and with a given subframe offset relative to the framestart.

If only one CSI process is used, then it is common to let the CSI-IMreflect the interference from all other eNBs, i.e., the serving celluses a ZP CSI-RS that overlaps with the CSI-IM, but in other adjacenteNBs there is no ZP CSI-RS on these resources. In this way the UE willmeasure the interference from adjacent cells using the CSI-IM.

If additional CSI processes are configured to the UE, than there ispossibility for the network to also configure a ZP CSI-RS resource inthe adjacent eNB that overlaps with a CSI-IM resource for this CSIprocess for the UE in the serving eNB. In this way the UE will feed backaccurate CSI also for the case when this adjacent cell is nottransmitting. Hence, coordinated scheduling between eNBs is enabled withthe use of multiple CSI processes and one CSI process feeds back CSI forthe full interference case and the other CSI process feeds back CSI forthe case when a (strongly interfering) adjacent cell is muted. Asmentioned above, up to four CSI processes can be configured to the UE,thereby enabling feedback of four different transmission hypotheses.

The PDCCH/EPDCCH is used to carry Downlink Control Information (DCI)such as scheduling decisions and power control commands. Morespecifically, the DCI includes:

-   -   DL scheduling assignments, including Physical Downlink Shared        Channel (PDSCH) resource indication, transport format, hybrid        Automatic Repeat Request (ARQ) information, and control        information related to spatial multiplexing (if applicable). A        DL scheduling assignment also includes a command for power        control of the Physical Uplink Control Channel (PUCCH) used for        transmission of hybrid ARQ acknowledgements in response to DL        scheduling assignments.    -   UL scheduling grants, including Physical Uplink Shared Channel        (PUSCH) resource indication, transport format, and hybrid ARQ        related information. A UL scheduling grant also includes a        command for power control of the PUSCH.    -   Power control commands for a set of terminals as a complement to        the commands included in the scheduling assignments/grants.

The PDCCH/EPDCCH region carries one or more DCI messages, each with oneof the formats above. As multiple terminals can be scheduledsimultaneously, on both DL and UL, there must be a possibility totransmit multiple scheduling messages within each subframe. Eachscheduling message is transmitted on separate PDCCH/EPDCCH physicalresources. Furthermore, to support different radio channel conditions,link adaptation can be used, where the code rate of the PDCCH/EPDCCH isselected by adapting the resource usage for the PDCCH/EPDCCH, to matchthe radio channel conditions.

Against this backdrop, future cellular communications networks areexpected to utilize beamforming where the number of beams may exceed thenumber of CSI-RS resources. In addition, existing and future cellularcommunications networks sometimes use a multi-layer radio access networkincluding a number of coverage cells (e.g., macro cells controlled byeNBs) and a number of capacity cells (e.g., pico cells controlled bypico eNBs). As such, there is a need for systems and methods that enableimproved CSI-RS configuration, particularly for cellular communicationsnetworks that utilize beamforming and/or multi-layer radio accessnetworks.

SUMMARY

Systems and methods relating to Channel State Information (CSI) feedbackin a cellular communications network are disclosed. While not beinglimited thereto, embodiments disclosed here are particularly well-suitedto improve CSI feedback in a cellular communications network thatutilizes beamformed CSI Reference Signals (CSI-RSs) such that the sameCSI-RS resource may be reused over time in different beams.

Embodiments of a method of operation of a base station of a cellularcommunications network to control CSI-RS based channel estimation at awireless device are disclosed. In some embodiments, the method ofoperation of the base station comprises disabling inter-subframe channelinterpolation and/or filtering of CSI-RS estimates across subframes atthe wireless device and receiving one or more CSI reports from thewireless device that are generated by the wireless device withinter-subframe channel interpolation and/or filtering of CSI-RSestimates across subframes disabled in response to the base stationdisabling inter-subframe channel interpolation and/or filtering ofCSI-RS estimates across subframes at the wireless device. In thismanner, CSI feedback is improved particularly in embodiments in whichthe base station transmits a beamformed CSI-RS resource(s) and reusesthe same CSI-RS resource(s) for different beams over time. In this case,without disabling inter-subframe channel interpolation and/or filteringof CSI-RS estimates across subframes, the wireless device may performinter-subframe channel interpolation and/or filtering of CSI-RSestimates on a particular CSI-RS resource that is transmitted ondifferent beams in different subframes, which in turn would result inless than optimal CSI feedback.

In some embodiments, the wireless device utilizes two or more CSIprocesses for CSI reporting, and disabling inter-subframe channelinterpolation and/or filtering of CSI-RS estimates across subframescomprises disabling inter-subframe channel interpolation and/orfiltering of CSI-RS estimates across subframes on a per CSI processbasis. In other embodiments, the wireless device utilizes two or moreCSI processes for CSI reporting, and disabling inter-subframe channelinterpolation and/or filtering of CSI-RS estimates across subframescomprises disabling inter-subframe channel interpolation and/orfiltering of CSI-RS estimates across subframes for all of the two ormore CSI processes.

In some embodiments, disabling inter-subframe channel interpolationand/or filtering of CSI-RS estimates across subframes comprisesdisabling inter-subframe channel interpolation and/or filtering ofCSI-RS estimates across subframes via Radio Resource Control (RRC)signaling. Further, in some embodiments, disabling inter-subframechannel interpolation and/or filtering of CSI-RS estimates acrosssubframes via RRC signaling comprises sending, in an RRC informationelement that configures a CSI process of the wireless device, anindication that inter-subframe channel interpolation and/or filtering ofCSI-RS estimates across subframes is not allowed for the CSI process ofthe wireless device.

In some embodiments, the method of operation of the base station furthercomprises disabling combining of CSI Interference Measurement (CSI-IM)estimates across subframes at the wireless device.

In some embodiments, disabling inter-subframe channel interpolationand/or filtering of CSI-RS estimates across subframes comprisessignaling, to the wireless device, an indication that inter-subframechannel interpolation and/or filtering of CSI-RS estimates acrosssubframes is not allowed.

In some embodiments, the method of operation of the base station furthercomprises configuring the wireless device with a set of CSI-RSresources. Further, in some embodiments, receiving the one or more CSIreports from the wireless device comprises receiving CSI reports for asubset of the set of CSI-RS resources configured for the wirelessdevice. In some embodiments, configuring the wireless device with theset of CSI-RS resources comprises configuring the wireless device withthe set of CSI-RS resources via RRC signaling. In other embodiments,configuring the wireless device with the set of CSI-RS resourcescomprises semi-statically configuring the wireless device with the setof CSI-RS resources. In some embodiments, the set of CSI-RS resources isspecific to a CSI process of the wireless device.

In some embodiments, the base station transmits beamformed CSI-RS, andthe method of operation of the base station further comprisesdynamically changing beams used on the set of CSI-RS resourcesconfigured for the wireless device.

Embodiments of a base station enabled to control CSI-RS based channelestimation at a wireless device are also disclosed. In some embodiments,the base station operates according to any of the embodiments of themethod of operation of a base station described herein.

Embodiments of a method of operation of a wireless device in a cellularcommunications network to provide CSI reporting are disclosed. In someembodiments, the method of operation of the wireless device comprisesreceiving an indication from a base station of the cellularcommunications network to disable inter-subframe channel interpolationand/or filtering of CSI-RS estimates across subframes and, in response,performing one or more CSI-RS measurements with inter-subframe channelinterpolation and/or filtering of CSI-RS estimates across subframesdisabled. The method further comprises transmitting a CSI report to thebase station based on the one or more CSI-RS measurements.

In some embodiments, the base station transmits a beamformed CSI-RSresource and reuses the same CSI-RS resource for different beams overtime.

In some embodiments, the wireless device utilizes two or more CSIprocesses for CSI reporting, and the indication received from the basestation is an indication to disable inter-subframe channel interpolationand/or filtering of CSI-RS estimates across subframes for a particularCSI process. In other embodiments, the wireless device utilizes two ormore CSI processes for CSI reporting, and the indication received fromthe base station is an indication to disable inter-subframe channelinterpolation and/or filtering of CSI-RS estimates across subframes forall of the two or more CSI processes.

In some embodiments, receiving the indication comprises receiving theindication via RRC signaling. In some embodiments, the wireless deviceutilizes two or more CSI processes for CSI reporting, the indicationreceived from the base station is an indication to disableinter-subframe channel interpolation and/or filtering of CSI-RSestimates across subframes for a particular CSI process of the wirelessdevice, and receiving the indication comprises receiving the indicationcomprised in an RRC information element that configures the particularCSI process of the wireless device.

In some embodiments, the method of operation of the wireless devicefurther comprises receiving, from the base station, an indication todisable combining of CSI-IM estimates across subframes and, in response,performing one or more CSI-IM measurements with combining of CSI-IMestimates across subframes disabled.

In some embodiments, the method of operation of the wireless devicefurther comprises receiving a configuration of a set of CSI-RS resourcesfor the wireless device. In some embodiments, the CSI report is for asubset of the set of CSI-RS resources configured for the wirelessdevice. In some embodiments, receiving the configuration of the set ofCSI-RS resources comprises receiving the configuration of the set ofCSI-RS resources from the base station via semi-static signaling (e.g.,RRC signaling). In some embodiments, the set of CSI-RS resources isspecific to a CSI process of the wireless device.

In some embodiments, the base station transmits beamformed CSI-RS, andbeams used on the set of CSI-RS resources configured for the wirelessdevice are dynamically changed.

Embodiments of a wireless device in a cellular communications network toprovide CSI reporting are disclosed. In some embodiments, the wirelessdevice operates according to any of the embodiments of the method ofoperation of a wireless device described herein.

Those skilled in the art will appreciate the scope of the presentdisclosure and realize additional aspects thereof after reading thefollowing detailed description of the embodiments in association withthe accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawing figures incorporated in and forming a part ofthis specification illustrate several aspects of the disclosure, andtogether with the description serve to explain the principles of thedisclosure.

FIG. 1 illustrates the LTE downlink physical resource;

FIG. 2 illustrates the LTE time-domain structure;

FIG. 3 illustrates a downlink subframe;

FIGS. 4A and 4B illustrate configurations of Channel State InformationReference Signal (CSI-RS) for different numbers of antenna ports;

FIG. 5 illustrates one example of a cellular communications networkimplementing flexible Channel State Information (CSI) feedback accordingto some embodiments of the present disclosure;

FIG. 6 illustrates the operation of the base station and the wirelessdevice of FIG. 5 according to some embodiments of the presentdisclosure;

FIG. 7 illustrates the operation of the base station and the wirelessdevice of FIG. 5 to provide disabling of inter-subframeinterpolation/filtering of CSI-RS estimates according to someembodiments of the present disclosure;

FIG. 8 illustrates the operation of the base station and the wirelessdevice of FIG. 5 to provide dynamic CSI feedback according to someembodiments of the present disclosure;

FIG. 9 illustrates the operation of the base station and the wirelessdevice of FIG. 5 to provide dynamic CSI feedback via dynamic CSI-RSresource configuration according to some embodiments of the presentdisclosure;

FIG. 10 illustrates the operation of the base station and the wirelessdevice of FIG. 5 to provide dynamic CSI feedback via dynamic CSI-RSresource configuration according to some other embodiments of thepresent disclosure;

FIG. 11 illustrates the operation of the base station and the wirelessdevice of FIG. 5 to provide dynamic CSI feedback via dynamic CSI-RSresource configuration using Downlink Control Information (DCI) messagesaccording to some other embodiments of the present disclosure;

FIG. 12 illustrates the operation of the base station and the wirelessdevice of FIG. 5 to provide dynamic CSI feedback via dynamic CSI-RSresource configuration using Long Term Evolution (LTE) Medium AccessControl (MAC) Control Elements (CEs) according to some other embodimentsof the present disclosure;

FIG. 13 illustrates the operation of the base station and the wirelessdevice of FIG. 5 to provide dynamic CSI feedback via dynamic Non-ZeroPower (NZP) CSI-RS and CSI Interference Measurement (CSI-IM) resourceconfiguration according to some embodiments of the present disclosure;

FIG. 14 illustrates the operation of the base station of FIG. 5 todynamically configure CSI-RS resources for the wireless device from aset of K CSI-RS resources transmitted on adjacent beams from theperspective of the base station according to some embodiments of thepresent disclosure;

FIG. 15 is a block diagram of the base station according to someembodiments of the present disclosure;

FIG. 16 is a block diagram of the base station according to otherembodiments of the present disclosure;

FIG. 17 is a block diagram of the wireless device according to someembodiments of the present disclosure; and

FIG. 18 is a block diagram of the wireless device according to otherembodiments of the present disclosure.

DETAILED DESCRIPTION

The embodiments set forth below represent information to enable thoseskilled in the art to practice the embodiments and illustrate the bestmode of practicing the embodiments. Upon reading the followingdescription in light of the accompanying drawing figures, those skilledin the art will understand the concepts of the disclosure and willrecognize applications of these concepts not particularly addressedherein. It should be understood that these concepts and applicationsfall within the scope of the disclosure and the accompanying claims.

Note that although terminology from 3^(rd) Generation PartnershipProgram (3GPP) Long-Term Evolution (LTE) has been used in thisdisclosure to exemplify the embodiments of the present disclosure, thisshould not be seen as limiting the scope of the concepts disclosedherein to only the aforementioned system. Other wireless systems,including Wideband Code Division Multiple Access (WCDMA), WiFi, WIMax,LTE for unlicensed band, Ultra Mobile Broadband (UMB), and Global Systemfor Mobile Communications (GSM), may also benefit from exploiting theideas covered within this disclosure.

Also note that terminology such as enhanced or evolved Node B (eNB) andUser Equipment (UE) should be considered as non-limiting and, inparticular, does not imply a certain hierarchical relationship betweenthe eNB and the UE. In general, “eNB” or “Transmission Point (TP)” couldbe considered as device 1 and “UE” as device 2, and these two devicescommunicate with each other over some radio channel. The disclosure alsofocuses on wireless transmissions in the downlink, but the disclosure isequally applicable in the uplink.

Before describing embodiments of the present disclosure, a discussion ofsome problems associated with conventional Channel State InformationReference Signal (CSI-RS) is beneficial. Some problems addressed in thisdisclosure relate to an eNB that transmits beamformed CSI-RSs, whereeach CSI-RS is associated with a certain, potentially narrow, beamtransmitted from, e.g., an array antenna. In other words, each CSI-RS istransmitted using a different precoder or different beamforming weights.

Existing Channel State Information (CSI) feedback solutions have severalproblems that are addressed in the present disclosure. Reconfigurationof the CSI-RS to measure on requires Radio Resource Control (RRC)signaling, which has two problems. First, there is a delay to establishthe reconfiguration, which may be up to 10 milliseconds (ms). Second, itis uncertain when the UE has adopted the reconfiguration, hence there isa period of uncertainty in system operation. Another problem withexisting CSI feedback solutions is that using multiple CSI processesrequires significant UE complexity, uplink signaling overhead, and powerconsumption, all of which are undesirable for network and UEimplementation.

Another problem is that, if beamformed CSI-RSs are used and the UE movesin a tangential direction as seen from the eNB, the CSI-RS on which theUE measures needs to be reconfigured often as the UE moves from the mainlobe of one beam to the main lobe of another beam. This problem isparticularly severe in case of high tangential UE velocity or narrowbeams from the eNB (i.e., a large number of horizontal array antennas).

The Physical Downlink Control Channel (PDCCH) and the Enhanced PDCCH(EPDCCH) can have relatively high block error rates, which means thatthe network may not know if a given Downlink Control Information (DCI)message is correctly received. Therefore, in case a DCI message changesa parameter used for periodic CSI reporting, the network may not know ifthe parameter contained in the DCI is used in subsequent periodic CSIreports since the UE may transmit periodic CSI reports using the sameformat and timing both before and after receiving (or not receiving) theDCI message.

Systems and methods relating to improved CSI feedback solutions that, atleast in some embodiments, address the problems described above aredisclosed. In some embodiments, an eNB indicates to a UE by higher layersignaling (e.g., RRC signaling) or in a DCI message that the UE is notallowed to perform channel interpolation of CSI-RS estimates acrosssubframes. In some embodiments, the eNB also indicates that averaging ofCSI-IM estimates is not allowed across subframes. In other words, theindication that the UE is not allowed to perform channel interpolationof CSI-RS estimates across subframes would ensure that no inter subframefiltering of Non-Zero-Power (NZP) CSI-RS based channel estimates isperformed for the purpose of CSI feedback on a CSI process. Thesignaling may further indicate the CSI processes for whichinter-subframe interpolation/filtering is disabled (e.g., predeterminedto be all or a subset of the possible CSI processes). In someembodiments, the RRC information element for configuring a CSI processmay be extended with a bit controlling whether inter-subframe NZP CSI-RSfiltering is enabled or disabled.

Disabling of inter-subframe interpolation/filtering of CSI-RS estimatesmay be particularly beneficial when CSI-RS resources are reused overtime between different beams transmitted by the eNB. In particular, whenthe eNB transmits beamformed CSI-RS and the number of beams is large(e.g., exceeds the number of CSI-RS resources available), then it may bebeneficial for the eNB to re-use the same CSI-RS resources on differentbeams over time. In other words, a particular CSI-RS resource may beused for a first beam during one subframe, and the same CSI-RS resourcemay be used for a second beam during another subframe. When re-usingCSI-RS resources over time between different beams, the performance ofconventional inter-subframe interpolation/filtering schemes utilized bythe UE generates CSI-RS measurements will result in poor CSI-RSestimates since measurements of different beams will be combined.Therefore, by disabling inter-subframe interpolation/filtering of CSI-RSestimates by the UE, the eNB improves the resulting CSI feedback as theeNB will know exactly which subframe and CSI-RS, and thus which beam, aparticular CSI feedback report is associated with.

In a further embodiment of disabled inter-subframe filtering, the UE ismonitoring a set of NZP CSI-RS configurations and selects a subset ofthose NZP CSI-RS configurations for reporting CSI. The selection could,for example, be based on estimates of channel strengths for themonitored NZP CSI-RS configurations (e.g., the subset could be selectedto correspond to the N strongest channels). The UE could be configured(via, e.g., a higher layer message) with such a monitoring set for eachof its CSI processes. The monitoring sets could be CSI process specific.With this embodiment; the network can now dynamically change the beamsused on a set of (periodically reoccurring) NZP CSI-RS resourcessignificantly larger than the current maximum number of NZP CSI-RSconfigurations currently handled by a single UE (which is three) withoutforcing the UE to handle the extra complexity of computing CSI for theentire monitoring set (computing channel strength is significantlysimpler than computing CSI).

In some embodiments, an eNB configures a UE with a set of K CSI-RSresources (also known as CSI-RS configurations) by higher layersignaling, e.g. by using an RRC message. The CSI-RSs are periodicallytransmitted, potentially with different periodicity.

In some embodiments, the K resources correspond to K different beamdirections as seen from the eNB. A typical number is K=20 since 20two-port CSI-RS can be transmitted in a single subframe according to LTEspecifications (3GPP TS 36.211 V12.3.0).

The eNB indicates to the UE, possibly in an uplink scheduling grantmessage or some other form of message (e.g., downlink assignment or adedicated message on a downlink control channel), a CSI-RS resource(/configuration) of the K CSI-RS resources (/configurations). This CSIresource is the RS for which the UE should perform channel measurements(hence the CSI-RS may be referred to as NZP CSI-RS) and use for at leastone subsequent CSI report. A CSI report transmitted on the uplink fromthe UE is then computed using measurements on the single CSI-RS out ofthe set of K possible CSI-RSs. Since a single CSI report and a singleCSI process is used, the UE complexity is reduced compared to usingmultiple CSI processes. In some embodiments, the signaling may take theform of associating the indicated CSI-RS resource (/configuration) witha CSI process, implying that CSI feedback for the corresponding CSIprocess would use the so associated NZP CSI-RS. In other embodiments,the indicated CSI-RS resource (/configuration) may be associated withmultiple CSI processes. In some embodiments, the same signaling messagemay contain multiple indications of associations between CSI-RSresources (/configurations) and CSI processes. In some embodiments, theassociation may hold for a single CSI reporting instance (e.g., oneassociated with the signaling message). If an additional CSI report isthereafter sent, the corresponding CSI process may revert back to usinga default CSI-RS resource (/configuration). Such a default CSI-RSresource may, for example, be represented by the semi-staticallyconfigured CSI configuration that is associated with the CSI processaccording to an LTE Release 11 RRC mechanism (for more information, see3GPP 36.331 V12.3.0). This may be the case for CSI on Physical UplinkShared Channel (PUSCH) (aperiodic) and/or Physical Uplink ControlChannel (PUCCH) (periodic). Alternatively, the dynamically signaledassociation between the CSI resource (/configuration) and the CSIprocess may hold until another association for that CSI process issignaled.

In some further embodiments, the eNB also indicates which one of the KCSI-RS resources that should be used as a CSI Interference Measurement(CSI-IM) resource.

In some embodiments, the UE assumes a Physical Downlink Shared Channel(PDSCH) rate matching around all K CSI-RS resources indicated in thehigher layer configuration.

In some further embodiments, periodic CSI reports using PUCCH arecomputed based on the CSI-RS resource indicated in a downlink DCImessage. The UE will use the selected CSI-RS resource for CSI feedbackuntil an indication of a new CSI-RS is received by the UE in a DCImessage. Additionally, the UE may provide an indication confirming whichCSI-RS resource is measured, the indication comprising an index of themeasured CSI-RS resource, or alternatively a bit confirming that thedownlink DCI message was successfully received and that the CSI-RSresource in the DCI message is used in the measurement.

In a further embodiment, periodic CSI reports using PUCCH are computedbased on the CSI-RS resource indicated in an LTE Medium Access Control(MAC) control element. The UE can be expected to use the CSI-RS resourceindicated in the MAC control element no later than a predeterminednumber of subframes after transmitting a Hybrid Automatic Repeat RequestAcknowledgement (HARQ-ACK) on PUCCH to the transport block containingthe MAC control element. In this way, the maximum length of theambiguity period in which the prior CSI-RS would be measured can bedetermined, and so subframes in which the CSI-RS resource indicated bythe MAC control element should be used for CSI reports can beidentified. Additionally or alternatively, the UE may provide anindication confirming which CSI-RS resource is measured; the indicationcomprising an index of the measured CSI-RS resource or alternatively abit confirming that the MAC control element was successfully receivedand that the CSI-RS resource is used in the measurement.

In some further embodiments of the eNB, the CSI resources configured tothe UE are transmitted in adjacent beams. Hence, the eNB can dynamicallychange the CSI measurement reports from the UE for the current beamserving the UE and for the neighboring beams of this serving beam.

As discussed above, embodiments of the present disclosure areimplemented in a cellular communications network 10, such as thatillustrated in FIG. 5. As illustrated, the cellular communicationsnetwork 10 includes a base station 12 (e.g., an eNB) and a wireless(e.g., a UE). Note that while the base station 12 is described asperforming some of the functionality disclosed herein, the concepts areequally applicable to any type of radio access node that desires toconfigure CSI measurements by the wireless device 14. The base station12 is connected to a core network (not shown).

FIG. 6 illustrates the operation of the base station 12 and the wirelessdevice 14 according to some embodiments of the present disclosure. Asdiscussed above, in some embodiments, the base station 12 disables theinter-subframe channel interpolation/filtering of the NZP CSI-RS and/oraveraging of CSI-IM belonging to a CSI process of the wireless device 14(step 100). Note that, in FIG. 6, step 100 is optional as indicated bythe dashed line. Notably, as will be appreciated by one of ordinaryskill in the art upon reading this disclosure, inter-subframeinterpolation and inter-subframe filtering are two different techniquesused for inter-subframe CSI-RS based channel estimation. Inter-subframeinterpolation uses CSI-RS estimates across subframes to interpolateadditional CSI-RS estimates. Conversely, inter-subframe filteringfilters, or averages, CSI-RS estimates across subframes. As such,“inter-subframe interpolation/filtering” refers to inter-subframeinterpolation and/or inter-subframe filtering. In some embodiments, thebase station 12 disables inter-subframe interpolation/filtering of NZPCSI-RS and/or averaging of CSI-IM in an uplink grant to the wirelessdevice 14. In some embodiments, the base station 12 accomplishes this inan RRC message to the wireless device 14. In some embodiments, whetheror not channel interpolation is done is encoded in an informationelement sent to the wireless device 14.

In some embodiments, the base station 12 configures the wireless device14 with a set of K CSI-RS resources by higher layer signaling, e.g. byusing an RRC message (step 102). Note that, in FIG. 6, step 102 isoptional as indicated by the dashed line. The base station 12 thendynamically configures one (or more) of the CSI-RS resources in the setof K CSI-RS resources to use for subsequent CSI feedback (step 104). Asused herein, a dynamic configuration is one that changes on a subframeor at least frame level (e.g., from one subframe to another). In step202, the base station 12 dynamically indicates which CSI-RS resource(s)that the wireless device 14 is to perform measurements on for subsequentCSI feedback. In some embodiments, the indication includes an indicationof at least one CSI-RS resource of the K CSI-RS resources to be used bythe wireless device 14. In some embodiments, this is accomplished withan uplink grant to wireless device 14.

The wireless device 14 then measures the indicated CSI-RS (step 106). Inother words, the wireless device 14 performs one or more measurements onthe one or more CSI-RS resources dynamically configured in step 104. Thewireless device 14 then reports the selected CSI-RS to the base station12 via a CSI report (step 110). In some embodiments, this is aperiodically scheduled CSI feedback. In some embodiments, this is anaperiodic CSI feedback.

FIGS. 7 through 13 illustrate various embodiments described above. Inparticular, FIG. 7 illustrates the operation of the base station 12 andthe wireless device 14 to enable disablement of inter-subframeinterpolation/filtering of NZP CSI-RS measurements according to someembodiments of the present disclosure. As illustrated, the base station12 disables inter-subframe channel interpolation/filtering of NZP CSI-RSat the wireless device 14 (step 200) Notably, in this embodiment,disabling of inter-subframe channel interpolation/filtering of NZPCSI-RS is performed on a per wireless device basis. The base station 12may disable inter-subframe channel interpolation/filtering of NZP CSI-RSat the wireless device 14 in response to some triggering event; e.g., anincrease in cell load, an increase in the number of beams transmitted bythe base station 12, etc. However, the triggering event can be anysuitable triggering event. The base station 12 disables inter-subframechannel interpolation/filtering of NZP CSI-RS at the wireless device 12by, in some embodiments, transmitting an indication to the wirelessdevice 14 that the wireless device 14 is not allowed to performinter-subframe channel interpolation/filtering of NZP CSI-RS. Thisindication may be transmitted using any suitable signaling such as, forexample, higher layer signaling (e.g., RRC signaling).

In some embodiments, the base station 12 disables inter-subframe channelinterpolation/filtering of NZP CSI-RS at the wireless device 14 for oneor more particular CSI processes of the wireless device 12. Forinstance, the base station 12 includes an indication that inter-subframechannel interpolation/filtering of NZP CSI-RS at the wireless device 14is to be disabled (i.e., not allowed) for a particular CSI processwithin an RRC information element used to configure that CSI process. Inthis manner, the base station 12 can separately disable or enableinter-subframe channel interpolation/filtering of NZP CSI-RS formultiple CSI processes configured for the wireless device 14. In otherembodiments, the base station 12 disables inter-subframe channelinterpolation/filtering of NZP CSI-RS at the wireless device 14 formultiple CSI processes (e.g.; two or more or even all CSI processes)using a single indicator.

Optionally, in some embodiments, the base station 12 may also disableaveraging of CSI-IM estimates at the wireless device (step 202).Notably, while averaging of CSI-IM estimates is described as beingdisabled in some of the embodiments disclosed herein, the presentdisclosure is not limited to averaging. Averaging is just one example ofhow CSI-IM estimates can be combined across subframes. As such, in thisregard, any combining (e.g., filtering or averaging) of multiple CSI-IMestimates across subframes may be disabled. In other words, the basestation 12 may also transmit an indication to the wireless device 14that indicates that the wireless device 14 is not to perform averagingof CSI-IM estimates. This indication may be provided via higher layersignaling, e.g., RRC signaling. As discussed above with respect to step200, the indication to disable averaging of CSI-IM estimates may beprovided separately for each of multiple CSI processes at the wirelessdevice 14 or a single indication may be used for multiple or even allCSI processes.

In response to receiving the indication from the base station 12 in step200, the wireless device 14 performs CSI-RS measurement(s) withinter-subframe channel interpolation/filtering of NZP CSI-RS disabled(step 204). Similarly, if averaging of CSI-IM estimates has beendisabled, the wireless device 14 performs CSI-IM measurement(s) withaveraging of CSI-IM estimates disabled (step 206). The wireless device14 then provides CSI feedback to the base station 12 via a CSI report(s)determined from the measurements (step 208). Notably, if there aremultiple CSI processes at the wireless device 14, then a separate CSIreport for each CSI process may be used to report the CSI feedback tothe base station 12. Also, if triggered (aperiodic) CSI reporting isused (e.g., using PUSCH in LTE), the wireless device 14 can sendmultiple CSI reports together (stacked) in a single message (e.g., asingle PUSCH message in LTE).

FIG. 8 illustrates the operation of the base station 12 and the wirelessdevice 14 according to some embodiments of the present disclosure inwhich the base station 12 configures the wireless device 14 with a setof K CSI-RS resources and the wireless device 14 selects a subset of theconfigured set of CSI-RS resources on which to measure for CSI feedback.In some embodiments, the process of FIG. 8 is utilized together with theprocess of FIG. 7 (i.e., inter-subframe channel interpolation/filteringcan be disabled for one or more or even all CSI processes).

As illustrated, the base station 12 configures the wireless device 14with one or more sets of K CSI-RS resources (step 300). Thisconfiguration is a static or semi-static configuration. For instance,this configuration may be made semi-statically via higher layersignaling such as, for example, RRC signaling. Further, a single set ofK CSI-RS resources may be configured for all CSI processes of thewireless device 14 (i.e., the same set of K CSI-RS resources is used forall CSI processes). However, in other embodiments, a separate set ofCSI-RS resources may be configured for each CSI process. In someparticular embodiments, the base station 12 transmits beamformed CSI-RS,and the set of K CSI-RS resources configured for a CSI process or allCSI processes corresponds to K different beam directions or beams asseen by the base station 12. In this case, K may be, for example, 20since 20 two-port CSI-RSs can be transmitted in a single subframe (3GPPTS 36.211 V12.3.0). Further, the K beams may include a serving beam ofthe wireless device 14 and a number of neighboring beams of the servingbeam of the wireless device 14.

The wireless device 14 then dynamically selects a subset of theconfigured set(s) of CSI-RS resources for CSI reporting (step 302). Inembodiments where a different set of CSI-RS resources is configured foreach CSI-RS process, for each CSI process, the wireless device 14dynamically selects a subset of the configured set of CSI-RS resourcesfor CSI reporting for that CSI process. This selection could, forexample, be based on estimates of channel strengths for the configuredCSI-RS resources (e.g., the subset could be selected to correspond tothe N strongest channels, where 0<N≤K). The wireless device 14 performsmeasurement(s) on the selected subset of the configured set(s) of CSI-RSresources (step 304) and provides CSI feedback based on the measurementsvia a CSI report(s) (step 306). Notably, the wireless device 14 mayinclude an indication of the selected CSI-RS resource(s) in the CSIreport(s) or provide such an indication to the base station 12 via aseparate message(s). Steps 302-306 may be repeated periodically (e.g.,for periodic CSI reporting) or aperiodically (for aperiodic CSIreporting). Conversely, the configuration of step 300 may be performedinfrequently or only once.

FIG. 9 illustrates the operation of the base station 12 and the wirelessdevice 14 to enable dynamic CSI reporting according to some embodimentsof the present disclosure. As illustrated, the base station 12configures the wireless device 14 with one or more sets of K CSI-RSresources (step 400). This configuration is a static or semi-staticconfiguration. For instance, this configuration may be madesemi-statically via higher layer signaling such as, for example, RRCsignaling. Further, a single set of K CSI-RS resources may be configuredfor all CSI processes of the wireless device 14 (i.e., the same set of KCSI-RS resources is used for all CSI processes). However, in otherembodiments, a separate set of CSI-RS resources may be configured foreach CSI process. In some particular embodiments, the base station 12transmits beamformed CSI-RS, and the set of K CSI-RS resourcesconfigured for a CSI process or all CSI processes corresponds to Kdifferent beam directions or beams as seen by the base station 12. Inthis case, K may be, for example, 20 since 20 two-port CSI-RSs can betransmitted in a single subframe (3GPP TS 36.211 V12.3.0). Further, theK beams may include a serving beam of the wireless device 14 and anumber of neighboring beams of the serving beam of the wireless device14.

After configuring the set(s) of K CSI-RS resources, the base station 12dynamically configures CSI-RS resource(s) for measurement from theset(s) of CSI-RS resources (step 402). This dynamic configuration isperformed by dynamically transmitting an indication(s) to the wirelessdevice 14 of which CSI-RS resource(s) from the set(s) of CSI-RSresources configured for the wireless device 14 are to be used formeasurement. In some embodiments, the dynamic configuration istransmitted via an uplink scheduling grant message, a downlinkassignment, a message on a dedicated control channel, a DCI message, ora LTE MAC Control Element (CE). The dynamic configuration is used for atleast one subsequent CSI report. In some embodiments, the dynamicconfiguration is to be used for only one subsequent CSI report. In otherembodiments, the dynamic configuration is to be used for CSI reportsuntil a new dynamic configuration is received.

After receiving the dynamic configuration, the wireless device 14performs measurement(s) on the dynamically configured CSI-RS resource(s)(step 404) and transmits a corresponding CSI report(s) to the basestation 12 (step 406). Notably, the wireless device 14 may include anindication of the CSI-RS resource(s) used for the CSI report(s) (or someother indication that the dynamically configured CSI-RS resource(s) wereused for the CSI report(s)) in the CSI report(s) or provide such anindication to the base station 12 via a separate message(s). The CSIreport(s) may be a periodic CSI report(s) or aperiodic CSI report(s). Asdiscussed above, the dynamic configuration is used for only one CSIreport. In this case, the process proceeds to step 412 discussed below.However, in other embodiments, the dynamic configuration applies until anew dynamic configuration is received. In this regard, the wirelessdevice 14 continues to perform measurement(s) and transmit correspondingCSI reports, periodically or aperiodically, until a new dynamicconfiguration is received (steps 408 and 410). Once a new dynamicconfiguration is transmitted by the base station 12 and received by thewireless device 14 (step 412), the wireless device 14 performsmeasurement(s) on the newly configured CSI-RS resource(s) and reportsthe corresponding CSI report(s) to the base station 12 (step 414 and416). The process continues in this manner.

Notably, in some embodiments when performing the measurements on thedynamically configured CSI-RS resource(s), the wireless device 14assumes PDSCH rate matching around CSI-RS and CRS, PDSCH rate matchingaround CSI-RS is, in particular, where PDSCH is not mapped to anyResource Elements (REs) in the union of the ZP and NZP CSI-RS resourcesin the configured set(s) of CSI-RS resources. In other words, whenmapping PDSCH to REs for transmission at the base station 12, the PDSCHis not mapped to any REs that are included in the ZP and NZP CSI-RSresources in the configured set(s) of CSI-RS resources.

FIG. 10 illustrates the operation of the base station 12 and thewireless device 14 according to some embodiments in which the CSI-RSresource configuration reverts back to some default configuration afterCSI reporting on a dynamically configured CSI-RS resource(s) iscomplete. In this example, steps 500-506 are the same as steps 400-406of FIG. 9 and, as such, the details are not repeated. After transmittingthe CSI report(s) based on the dynamically configured CSI-RS resource(s)in step 506, rather than continuing to report using the same dynamicallyconfigured CSI-RS resources, the wireless device 14 reverts to a defaultCSI-RS resource(s). In particular, the wireless device 14 performsmeasurement(s) on the default CSI-RS resource(s) (step 508) andtransmits a corresponding CSI report(s) to the base station 12 (step510). The CSI report(s) may be a periodic CSI report(s) or aperiodic CSIreport(s). In this example, the wireless device 14 continues to performmeasurement(s) and transmit corresponding CSI reports, periodically oraperiodically, based on the default CSI-RS resource(s) until a newdynamic configuration is transmitted by the base station 12 and receivedby the wireless device 14 (step 512). Once a new dynamic configurationis received by the wireless device 14, the wireless device 14 performsmeasurement(s) on the newly configured CSI-RS resource(s) and reportsthe corresponding CSI report(s) to the base station 12 (steps 514 and516). The process continues in this manner. Notably, in some embodimentswhen performing the measurements on the dynamically configured CSI-RSresource(s), the wireless device 14 assumes PDSCH rate matching aroundCSI-RS and CRS.

FIG. 11 illustrates an embodiment in which the CSI-RS resource(s) isdynamically configured via a DCI message. Using the DCI message for thedynamic configuration may be particularly well-suited for aperiodic CSIreporting, but is not limited thereto. As illustrated, the base station12 configures the wireless device 14 with one or more sets of K CSI-RSresources (step 600). As discussed above, this configuration is a staticor semi-static configuration. For instance, this configuration may bemade semi-statically via higher layer signaling such as, for example,RRC signaling. Further, a single set of K CSI-RS resources may beconfigured for all CSI processes of the wireless device 14 (i.e., thesame set of K CSI-RS resources is used for all CSI processes). However,in other embodiments, a separate set of CSI-RS resources may beconfigured for each CSI process. In some particular embodiments, thebase station 12 transmits beamformed CSI-RS, and the set of K CSI-RSresources configured for a CSI process or all CSI processes correspondsto K different beam directions or beams as seen by the base station 12.In this case, K may be, for example, 20 since 20 two-port CSI-RSs can betransmitted in a single subframe (3GPP TS 36.211 V12.3.0). Further, theK beams may include a serving beam of the wireless device 14 and anumber of neighboring beams of the serving beam of the wireless device14.

After configuring the set(s) of K CSI-RS resources, the base station 12dynamically configures CSI-RS resource(s) for measurement from theset(s) of CSI-RS resources via a DCI message (step 602). The DCI messageincludes an indication of configured CSI-RS resource(s) from theconfigured set(s) of CSI-RS resources (e.g., an index or indices). Afterreceiving the dynamic configuration, the wireless device 14 performsmeasurement(s) on the dynamically configured CSI-RS resource(s) (step604) and transmits a corresponding CSI report(s) to the base station 12(step 606), as discussed above. While not being limited hereto, in thisexample, the wireless device 14 continues to use the same dynamicallyconfigured CSI-RS resource(s) for one or more subsequent CSI reports(not shown). This may be the case where, for example, CSI reporting isperiodic. However, in other embodiments, CSI reporting is aperiodic andthe CSI-RS resource(s) to use may be dynamically configured for eachaperiodic CSI report, for example. Once a new dynamic configuration istransmitted by the base station 12 and received by the wireless device14 (step 612), the wireless device 14 performs measurement(s) on thenewly configured CSI-RS resource(s) and reports the corresponding CSIreport(s) to the base station 12 (steps 614 and 616). The processcontinues in this manner. Notably, in some embodiments when performingthe measurements on the dynamically configured CSI-RS resource(s), thewireless device 14 assumes PDSCH rate matching around CSI-RS.

FIG. 12 illustrates an embodiment in which the CSI-RS resource(s) isdynamically configured via a LTE MAC CE. In this particular example, CSIreporting is periodic; however, the present disclosure is not limitedthereto. As illustrated, the base station 12 configures the wirelessdevice 14 with one or more sets of K CSI-RS resources (step 700). Asdiscussed above, this configuration is a static or semi-staticconfiguration. For instance, this configuration may be madesemi-statically via higher layer signaling such as, for example, RRCsignaling. Further, a single set of K CSI-RS resources may be configuredfor all CSI processes of the wireless device 14 (i.e., the same set of KCSI-RS resources is used for all CSI processes). However, in otherembodiments, a separate set of CSI-RS resources may be configured foreach CSI process. In some particular embodiments, the base station 12transmits beamformed CSI-RS, and the set of K CSI-RS resourcesconfigured for a CSI process or all CSI processes corresponds to Kdifferent beam directions or beams as seen by the base station 12. Inthis case, K may be, for example, 20 since 20 two-port CSI-RSs can betransmitted in a single subframe (3GPP TS 36.211 V12.3.0). Further, theK beams may include a serving beam of the wireless device 14 and anumber of neighboring beams of the serving beam of the wireless device14.

After configuring the set(s) of K CSI-RS resources, the base station 12dynamically configures CSI-RS resource(s) for measurement from theset(s) of CSI-RS resources via a LTE MAC CE (step 702). The LTE MAC CEincludes an indication of configured CSI-RS resource(s) from theconfigured set(s) of CSI-RS resources (e.g., an index or indices). Inresponse to receiving the dynamic configuration, the wireless device 14sends an acknowledgement (e.g., a HARQ-ACK) to the base station 12 toconfirm receipt of the transport block containing the LTE MAC CE (step704).

There is a certain amount of time that it takes the wireless device 14to effect the dynamic configuration of the CSI-RS resources, that is tobegin measuring and reporting CSI measurements of the CSI-RS resources.Particularly for periodic CSI reporting, this results in a time ofambiguity in which any CSI reports received from the wireless device 14are inaccurate (i.e., are based on measurements on the previouslyconfigured CSI-RS resource(s) rather than the newly configured CSI-RSresource(s)). As such, in this example, the base station 12 discards anyCSI reports received from the wireless device 14 during a predefinedamount of time after receiving the acknowledgement from the wirelessdevice 14 in step 704 (steps 706 and 708). This predefined amount oftime is greater than or equal to the amount of time that it takes forthe wireless device 14 to effect the dynamic configuration of the CSI-RSresource received in step 702.

In response to receiving the dynamic configuration in step 702, thewireless device 14 performs measurement(s) on the dynamically configuredCSI-RS resource(s) (step 710) and transmits a corresponding CSIreport(s) to the base station 12 (step 712), as discussed above.Notably, the time needed by the wireless device 14 to perform themeasurement(s) in step 710 may be part of the time of ambiguitydiscussed above. Notably, the wireless device 14 may include anindication of the CSI-RS resource(s) used for the CSI report(s) (or someother indication that the dynamically configured CSI-RS resource(s) wereused for the CSI report(s)) in the CSI report(s) or provide such anindication to the base station 12 via a separate message(s). In thiscase, now that the predefined amount of time since receiving theacknowledgement in step 704 has expired, the base station 12 can becertain that the CSI report is based on the CSI-RS resource(s)configured in step 702. As such, the base station 12 processes the CSIreport(s) (e.g., to select transmission parameters for the downlink tothe wireless device 14 in the conventional manner) (step 714). Notably,in some embodiments, the CSI report(s) include an indication that thatthe dynamically configured CSI-RS resource(s) were used for the CSIreport(s). This indication may be, for example, an indication (e.g., anindex) of the CSI-RS resource(s) used for the CSI report(s) or any othersuitable indication.

At this point, as discussed above, the wireless device 14 may continueto use the same dynamically configured CSI-RS resource(s) for one ormore subsequent CSI reports (not shown). This may be the case where, forexample, CSI reporting is periodic. However, in other embodiments, CSIreporting is aperiodic, and the CSI-RS resource(s) to use may bedynamically configured for each aperiodic CSI report, for example. Inother embodiments, the wireless device 14 may revert to some defaultCSI-RS resource(s) until a new dynamic configuration is received.Notably, in some embodiments when performing the measurements on thedynamically configured CSI-RS resource(s), the wireless device 14assumes PDSCH rate matching around CSI-RS.

While the embodiments described above with respect to FIGS. 9-12 focuson dynamic configuration of CSI-RS resources in general, it should benoted that these CSI-RS resources are, in some embodiments, NZP CSI-RSresources and, in other embodiments, NZP CSI-RS resources and/or CSI-IMresources. In this regard, FIG. 13 illustrates the operation of the basestation 12 to dynamically configure NZP CSI-RS resource(s) and CSI-IMresource(s) according to some embodiments of the present disclosure. Asillustrated, the base station 12 configures the wireless device 14 withone or more sets of K CSI-RS resources (step 800). As discussed above,this configuration is a static or semi-static configuration. Forinstance, this configuration may be made semi-statically via higherlayer signaling such as, for example, RRC signaling. Further, a singleset of K CSI-RS resources may be configured for all CSI processes of thewireless device 14 (i.e., the same set of K CSI-RS resources is used forall CSI processes). However, in other embodiments, a separate set ofCSI-RS resources may be configured for each CSI process. In someparticular embodiments, the base station 12 transmits beamformed CSI-RS,and the set of K CSI-RS resources configured for a CSI process or allCSI processes corresponds to K different beam directions or beams asseen by the base station 12. In this case, K may be, for example, 20since 20 two-port CSI-RSs can be transmitted in a single subframe (3GPPTS 36.211 V12.3.0). Further, the K beams may include a serving beam ofthe wireless device 14 and a number of neighboring beams of the servingbeam of the wireless device 14. In some embodiments, the sets of CSI-RSresources include a first set of NZP CSI-RS resources and a second setof CSI-IM resources (which may also be referred to as ZP CSI-RSresources).

After configuring the set(s) of K CSI-RS resources, the base station 12dynamically configures NZP CSI-RS resource(s) and CSI-IM resource(s) formeasurement from the set(s) of CSI-RS resources (step 802). The detailsfor the dynamic configuration are as discussed above. For instance, thedynamic configuration may include different NZP CSI-RS resources andCSI-IM resources for each of two or more CSI processes. In other words,the dynamic configuration may be CSI process specific. Further, thedynamic configuration may be performed by transmitting an appropriateindication(s) in, e.g., a DCI message or a LTE MAC CE, for example. Insome embodiments, the set(s) of CSI-RS resources configured in step 800include a set of NZP CSI-RS resources and a set of CSI-IM resources.Then, in step 802, the base station 12 dynamically configures one ormore NZP CSI-RS resources for measurement from the set of NZP CSI-RSresources (e.g., one NZP CSI-RS resource for each CSI process) and oneor more CSI-IM resources for interference measurement from the set ofCSI-IM resources (e.g., one CSI-IM resource for each CSI process).

In response to receiving the dynamic configuration, the wireless device14 performs measurement(s) on the dynamically configured NZP CSI-RSresource(s) and CSI-IM resource(s) (step 804). The measurement(s) on theNZP CSI-RS resource(s) is measurement(s) of a desired signal, whereasthe measurement(s) on the CSI-IM resource(s) is measurement(s) ofinterference, as will be appreciated by one of ordinary skill in the artupon reading this disclosure. The wireless device 14 transmits acorresponding CSI report(s) to the base station 12 based on themeasurements on the dynamically configured CSI-RS resource(s) and CSI-IMresource(s) (step 806). Notably, the wireless device 14 may include anindication of the NZP CSI-RS and CSI-IM resource(s) used for the CSIreport(s) (or some other indication that the dynamically configured NZPCSI-RS and CSI-IM resource(s) were used for the CSI report(s)) in theCSI report(s) or provide such an indication to the base station 12 via aseparate message(s).

At this point, as discussed above, the wireless device 14 may continueto use the same dynamically configured NZP CSI-RS resource(s) and CSI-IMresource(s) for one or more subsequent CSI reports (not shown). This maybe the case where, for example, CSI reporting is periodic. However, inother embodiments, CSI reporting is aperiodic, and the CSI-RSresource(s) to use may be dynamically configured for each aperiodic CSIreport, for example. In other embodiments, the wireless device 14 mayrevert to some default CSI-RS resource(s) until a new dynamicconfiguration is received. Notably, in some embodiments when performingthe measurements on the dynamically configured CSI-RS resource(s), thewireless device 14 assumes PDSCH rate matching around CSI-RS.

As discussed above, in some embodiments, the set of K CSI-RS resourcesconfigured for the wireless device 14 correspond to K different beamdirections from the perspective of the base station 12. Further, in someembodiments, the K different beam directions include the beam directionof a serving beam of the wireless device 14 and beam directions of anumber of neighboring beams of the serving beam of the wireless device14. The base station 12 can then dynamically configure (andre-configure) the CSI-RS resource(s) for measurement at the wirelessdevice 14 as the wireless device 14 transitions from one beam to another(i.e., as the serving beam of the wireless device 14 changes). In thisregard, FIG. 14 illustrates the operation of the base station 12according to some embodiments of the present disclosure. As illustrated,the base station 12 configures the wireless device 14 with a set of KCSI-RS resources (step 900). The configuration may be performed, asdiscussed above, via higher layer signaling (e.g., RRC signaling).Further, the set of K CSI-RS resources may be for multiple CSI processes(e.g., all CSI processes configured for the wireless device 14) or for asingle CSI process (e.g., a separate set of CSI-RS resources may beconfigured for each CSI process). Here, the base station 12 transmitsbeamformed CSI-RSs, and the set of K CSI-RS resources are transmitted ofK adjacent beams. The adjacent beams include a serving beam of thewireless device 14 and a number of neighboring beams of the serving beamof the wireless device 14.

The base station 12 dynamically configures CSI-RSs for measurement andCSI reporting for the serving beam of the wireless device 14 and one ormore neighboring beams from the set of CSI-RS resources (step 902). Thedynamic configuration may be performed via any suitable mechanism suchas, for example, a DCI message or a LTE MAC CE. The base station 12 mayconfigure one of the set of CSI-RS resources as a NZP CSI-RS formeasurement on the serving beam and one or more other CSI-RS resourcesfrom the set as CSI-IM resources for interference measurement. TheCSI-RS resources may continue to be dynamically configured such thatdifferent CSI-RS resources are configured for measurement andinterference measurement as the wireless device 14 transitions from onebeam to another.

FIG. 15 is a block diagram of the base station 12 (e.g., eNB) accordingto some embodiments of the present disclosure. As illustrated, the basestation 12 includes a baseband unit 16 including at least one processor18 (e.g., a Central Processing Unit (CPU), Application SpecificIntegrated Circuit (ASIC), Field-Programmable Gate Array (FPGA), etc.),memory 20, and a network interface 22 as well as a radio unit 24including a wireless, or Radio Frequency (RF), transceiver 26 thatincludes one or more transmitters 28 and one or more receivers 30coupled to one or more antennas 32. In some embodiments, thefunctionality of the base station 12 described herein is implemented insoftware that is stored in the memory 20 and executed by the at leastone processor 18, whereby the base station 12 operates to, e.g.,configure the set of CSI-RS resources for the wireless device 14,configure the measurement purposes of at least some and possibly all ofthe CSI-RS resources in the configured set, etc.

In some embodiments, a computer program is provided, where the computerprogram comprises instructions which, when executed on at least oneprocessor (e.g., the at least one processor 18), cause the at least oneprocessor to carry out the functionality of the base station 12according to any of the embodiments described herein. In someembodiments, a carrier containing the computer program is provided,wherein the carrier is one of an electronic signal, an optical signal, aradio signal, or a computer readable storage medium (e.g., anon-transitory computer readable medium).

FIG. 16 illustrates the base station 12 according to another embodimentof the present disclosure. As illustrated, the base station 12 includesa disabling module 34 (only in some embodiments) and a CSI-RS indicationmodule 36 (only in some embodiments), each of which is implemented insoftware. The disabling module 34 operates to disable inter-subframechannel interpolation/filtering of the NZP CSI-RS and/or averaging ofCSI-IM belonging to a CSI process for the wireless device 14 by, e.g.,transmitting an appropriate message(s) or signal(s) via an associatedtransmitter of the base station 12, as discussed above. The CSI-RSindication module 36 operates to indicate to the wireless device 14which CSI-RSs to measure by, e.g., transmitting an appropriatemessage(s) or signal(s) via an associated transmitter of the basestation 12. As discussed above, the indication of the CSI-RS resourceson which the wireless device 14 is to measure may be provided by firstconfiguring the wireless device 14 with a static or semi-static set ofCSI-RS resources (e.g., via RRC signaling) and then dynamicallyconfiguring which of the CSI-RS resources that the wireless device 14 isto measure on, e.g., via a DCI message or LTE MAC CEs.

FIG. 17 is a block diagram of the wireless device 14 according to someembodiments of the present disclosure. As illustrated, the wirelessdevice 14 includes at least one processor 40, memory 42, and a wireless,or RF, transceiver 44 that includes one or more transmitters 46 and oneor more receivers 48 coupled to one or more antennas 50. In someembodiments, the functionality of the wireless device 14 describedherein is implemented in software that is stored in the memory 42 andexecuted by the at least one processor 40.

In some embodiments, a computer program is provided, where the computerprogram comprises instructions which, when executed on at least oneprocessor (e.g., the at least one processor 40), cause the at least oneprocessor to carry out the functionality of the wireless device 14according to any of the embodiments described herein. In someembodiments, a carrier containing the computer program is provided,wherein the carrier is one of an electronic signal, an optical signal, aradio signal, or a computer readable storage medium (e.g., anon-transitory computer readable medium).

FIG. 18 illustrates the wireless device 14 according to some otherembodiments of the present disclosure. As illustrated, the wirelessdevice 14 includes a CSI-RS indication module 52, a metric computationmodule 54, and a reporting module 56, each of which is implemented insoftware. The CSI-RS indication module 52 operates to receive anindication of which CSI-RSs to measure via a receiver(s) of the wirelessdevice 14 (not shown). As discussed above, the CSI-RS indication module52 may first receive a static or semi-static configuration of a set ofCSI-RS resources (e.g., one set of CSI resources per CSI process or oneset of CSI-RS resources for multiple CSI processes). Then, the CSI-RSindication module 52 receives, via a receiver(s) of the wireless device14 (not shown), a dynamic configuration of which of the CSI-RS resourcesin the configured set(s) of CSI-RS resources that the wireless device 14is to measure on for CSI reporting. The metric computation module 54then computes a measurement(s) on the dynamically configured CSI-RSresource(s). The reporting module 56 then transmits a CSI report to thenetwork (e.g., to the base station 12) based on the measurement(s) viaan associated transmitter (not shown) of the wireless device 14.

Embodiments of systems and methods for flexible CSI feedback aredisclosed. In some embodiments, a network node (e.g., a radio accessnode such as, but not limited to, a base station) indicates to awireless device (e.g., a UE) which CSI-RS resource to measure. In someembodiments, this is accomplished with an uplink grant to the wirelessdevice.

In one embodiment, the base station configures the wireless device witha set of K CSI-RS resources by higher layer signaling, e.g. by using anRRC message. The base station then indicates to the wireless device,possibly in an uplink scheduling grant message or some other form ofmessage (e.g., downlink assignment, MAC CE, or a dedicated message on adownlink control channel), at least one CSI-RS resource of the K CSI-RSresources to be used by the wireless device. This at least one CSI-RSresource is the CSI-RS resource for which the UE should perform channelmeasurements. The wireless device then computes measurements on the atleast one CSI-RS resource out of the set of K possible CSI-RS resources.In some embodiments, the K CSI-RS resources may correspond to Kdifferent beam directions as seen from the base station. In oneembodiment, K=20 since 20 two-port CSI-RSs can be transmitted in asingle subframe.

In some embodiments, the network node also indicates to the wirelessdevice that the wireless device should disable the inter-subframechannel interpolation/filtering of the NZP CSI-RS belonging to a CSIprocess prior to indicating to the wireless device which CSI-RS resourceto measure. In some embodiments, this is accomplished via higher layersignaling such as RRC signaling or via a DCI message. In someembodiments, the base station also indicates that the averaging ofCSI-IM estimates is not allowed across subframes. In some embodiments,the signaling may further indicate for which CSI processes (e.g.,predetermined to be all or a subset of the possible CSI processes) thisapplies. In some embodiments, the RRC information element forconfiguring a CSI process may be extended with a bit controlling whetherinter-subframe NZP CSI-RS filtering is enabled or disabled.

In some embodiments, the wireless device then measures the indicatedCSI-RS. The wireless device then reports the selected CSI-RS to the basestation. In some embodiments, this is a periodically scheduled CSIfeedback. In some embodiments, this is an aperiodic CSI feedback. Insome embodiments, the aperiodic request is sent in an uplink grant.

In some embodiments, the wireless device is monitoring a set of NZPCSI-RS configurations and selects a subset of those NZP CSI-RSconfigurations for reporting CSI. In some embodiments, the selectioncould, for example, be based on estimates of channel strengths for themonitored NZP CSI-RS configurations (e.g., the subset could be selectedto correspond to the N strongest channels).

In some embodiments, the base station also indicates which one of theCSI-RS resources should be used as a CSI-IM resource. In someembodiments, the wireless device shall assume PDSCH rate matching aroundall CSI-RS resources indicated in the higher layer configuration.

In some embodiments, periodic CSI reports using PUCCH are computed basedon the CSI-RS resource indicated in a downlink DCI message. The wirelessdevice will use the selected CSI-RS resource for CSI feedback until anindication of a new CSI-RS is received by the wireless device in a DCImessage. Additionally, the wireless device may provide an indicationconfirming which CSI-RS resource is measured, the indication comprisingan index of the measured CSI-RS resource or alternatively a bitconfirming that the downlink DCI message was successfully received andthat the CSI-RS resource in the DCI message is used in the measurement.

In some embodiments, periodic CSI reports using PUCCH are computed basedon the CSI-RS resource indicated in an LTE MAC CE. In some embodiments,the wireless device may provide an indication confirming which CSI-RSresource is measured, the indication comprising an index of the measuredCSI-RS resource or alternatively a bit confirming that the MAC CE wassuccessfully received and that the CSI-RS resource is used in themeasurement.

In some embodiments, the CSI resources configured to the wireless deviceare transmitted in adjacent beams. Hence, the base station candynamically change the CSI measurement reports from the wireless devicefor the current beam serving the wireless device and for the neighboringbeams of this serving beam.

Embodiments of systems and methods for CSI feedback are disclosed. Inone embodiment, a method for CSI feedback, which is dynamic, has low UEcomplexity and solves the problems mentioned above:

-   -   A message is signaled from an eNB to the UE so that the UE        disables the inter-subframe channel interpolation/filtering of        the NZP CSI-RS belonging to a CSI process.    -   A dynamically signaled message (e.g., the uplink grant that        schedules an (aperiodic) CSI report) contains an indicator for        which CSI-RS resource the UE shall perform measurements on for a        subsequent aperiodic CSI feedback transmitted on PUSCH.        -   Since the uplink grant is delivered by layer 1 and because            the UE only transmits the aperiodic report when triggered to            do so, there is no uncertainty on when the UE has received            the indication.    -   After the CSI-RS resource indicator carried by DCI has been        received, the following periodic CSI reports transmitted using        PUCCH will be based on measurements on the indicated CSI-RS,        -   A confirmation indicator of the CSI-RS resource may be            included in the periodic CSI-RS report to validate that the            DCI was received, and that the measured CSI-RS resource is            the one carried by the DCI.

Embodiments of the CSI feedback framework disclosed herein have largebenefits over the LTE CSI framework when operating in an environmentwhere CSI-RS need to be reconfigured often as in the case of many smallcells or narrow beams and medium to high UE mobility.

The following acronyms are used throughout this disclosure.

-   -   μs Microsecond    -   2D Two-Dimensional    -   3GPP 3^(rd) Generation Partnership Project    -   ACK Acknowledgement    -   ABS Almost Blank Subframe    -   AP Antenna Port    -   ARQ Automatic Repeat Request    -   ASIC Application Specific Integrated Circuit    -   CDM Code-Division Multiplexing    -   CE Control Element    -   CFI Control Format Indicator    -   CoMP Coordinated Multipoint    -   CPU Central Processing Unit    -   CQI Channel Quality Information    -   CRS Cell-Specific Reference Symbol    -   CSI Channel State Information    -   CSI-RS Channel State Information Reference Signal    -   DCI Downlink Control Information    -   DFT Discrete Fourier Transform    -   DL Downlink    -   eNB Enhanced or Evolved Node B    -   EPDCCH Enhanced Physical Downlink Control Channel    -   FPGA Field-Programmable Gate Array    -   GSM Global System for Mobile Communications    -   HARQ Hybrid Automatic Repeat Request    -   ID Identifier    -   IM Interference Measurement    -   LTE Long Term Evolution    -   MAC Medium Access Control    -   ms Millisecond    -   NZP Non-Zero-Power    -   PDCCH Physical Downlink Control Channel    -   PDSCH Physical Downlink Shared Channel    -   PMI Precoding Matrix Indicator    -   PRB Physical Resource Block    -   PUCCH Physical Uplink Control Channel    -   PUSCH Physical Uplink Shared Channel    -   OFDM Orthogonal Frequency Division Multiplexing    -   QPSK Quadrature Phase Shift Keying    -   RB Resource Block    -   RE Resource Element    -   RF Radio Frequency    -   RI Rank Indicator    -   RPSF Reduced Power Subframe    -   RRC Radio Resource Control    -   SF Subframe    -   TM9 Transmission Mode 9    -   TM10 Transmission Mode 10    -   TS Technical Specification    -   TP Transmission Point    -   UE User Equipment    -   UL Uplink    -   UMB Ultra Mobile Broadband    -   WCDMA Wideband Code Division Multiple Access    -   ZP Zero-Power

Those skilled in the art will recognize improvements and modificationsto the embodiments of the present disclosure. All such improvements andmodifications are considered within the scope of the concepts disclosedherein.

What is claimed is:
 1. A method of operation of a base station of a cellular communications network to control Channel State Information Reference Symbol, CSI-RS, based channel estimation at a wireless device, comprising: sending an indication to the wireless device to disable inter-subframe channel interpolation of CSI-RS estimates across subframes; sending an indication to the wireless device to disable combining of Channel State Information Interference Measurement, CSI-IM, estimates across subframes; and receiving one or more Channel State Information, CSI, reports from the wireless device that are generated by the wireless device with disabled inter-subframe channel interpolation of CSI-RS estimates across subframes.
 2. The method of claim 1 wherein the base station transmits a beamformed CSI-RS resource, the beamformed CSI-RS resource corresponding to a beam direction, and reuses the same CSI-RS resource for different beam directions over time.
 3. The method of claim 1 wherein the wireless device utilizes two or more CSI processes for CSI reporting, and disabling inter-subframe channel interpolation of CSI-RS estimates across subframes comprises disabling inter-subframe channel interpolation of CSI-RS estimates across subframes on a per CSI process basis.
 4. The method of claim 1 wherein the wireless device utilizes two or more CSI processes for CSI reporting, and disabling inter-subframe channel interpolation of CSI-RS estimates across subframes comprises disabling inter-subframe channel interpolation of CSI-RS estimates across subframes for all of the two or more CSI processes.
 5. The method of claim 1 wherein disabling inter-subframe channel interpolation of CSI-RS estimates across subframes comprises disabling inter-subframe channel interpolation of CSI-RS estimates across subframes via Radio Resource Control, RRC, signaling.
 6. The method of claim 5 wherein disabling inter-subframe channel interpolation of CSI-RS estimates across subframes via RRC signaling comprises: sending, in an RRC information element that configures a CSI process of the wireless device, an indication that inter-subframe channel interpolation of CSI-RS estimates across subframes is not allowed for the CSI process of the wireless device.
 7. The method of claim 1 wherein disabling inter-subframe channel interpolation of CSI-RS estimates across subframes comprises signaling, to the wireless device, an indication that inter-subframe channel interpolation of CSI-RS estimates across subframes is not allowed.
 8. The method of claim 1 further comprising configuring the wireless device with a set of CSI-RS resources.
 9. The method of claim 8 wherein receiving the one or more CSI reports from the wireless device comprises receiving CSI reports for a subset of the set of CSI-RS resources configured for the wireless device.
 10. The method of claim 8 wherein configuring the wireless device with the set of CSI-RS resources comprises configuring the wireless device with the set of CSI-RS resources via Radio Resource Control, RRC, signaling.
 11. The method of claim 8 wherein configuring the wireless device with the set of CSI-RS resources comprises semi-statically configuring the wireless device with the set of CSI-RS resources.
 12. The method of claim 8 wherein the set of CSI-RS resources is specific to a CSI process of the wireless device.
 13. The method of claim 8 wherein the base station transmits beamformed CSI-RS, and the method further comprises dynamically changing beams used on the set of CSI-RS resources configured for the wireless device, each beam corresponding to a different beam direction.
 14. A base station of a cellular communications network enabled to control Channel State Information Reference Symbol, CSI-RS, based channel estimation at a wireless device, comprising: at least one transmitter; at least one receiver; at least one processor; and memory storing software instructions executable by the at least one processor whereby the base station is operative to: send an indication to the wireless device to disable, via the at least one transmitter, inter-subframe channel interpolation of CSI-RS estimates across subframes at a wireless device; send an indication to the wireless device to disable, via the at least one transmitter, combining of Channel State Information Interference Measurement, CSI-IM, estimates across subframes at the wireless device; and receive, via the at least one receiver, a Channel State Information, CSI, report from the wireless device that is generated by the wireless device with inter-subframe channel interpolation of CSI-RS estimates across subframes disabled in response to disabling inter-subframe channel interpolation of CSI-RS estimates across subframes at the wireless device.
 15. A method of operation of a wireless device in a cellular communications network to provide Channel State Information, CSI, reporting, comprising: receiving an indication from a base station of the cellular communications network to disable inter-subframe channel interpolation of Channel State Information Reference Symbol, CSI-RS, estimates across subframes; receiving an indication from the base station of the cellular communications network to disable combining of CSI Interference Measurement, CSI-IM, estimates across subframes; in response, performing one or more CSI-RS measurements with inter-subframe channel interpolation of CSI-RS estimates across subframes disabled; and transmitting a CSI report to the base station determined from the one or more CSI-RS measurements.
 16. The method of claim 15 wherein the base station transmits a beamformed CSI-RS resource and reuses the same CSI-RS resource for different beams over time.
 17. The method of claim 15 wherein the wireless device utilizes two or more CSI processes for CSI reporting, and the indication received from the base station is an indication to disable inter-subframe channel interpolation of CSI-RS estimates across subframes for a particular CSI process.
 18. The method of claim 15 wherein the wireless device utilizes two or more CSI processes for CSI reporting, and the indication received from the base station is an indication to disable inter-subframe channel interpolation of CSI-RS estimates across subframes for all of the two or more CSI processes.
 19. The method of claim 15 wherein receiving the indication comprises receiving the indication via Radio Resource Control, RRC, signaling.
 20. The method of claim 19 wherein the wireless device utilizes two or more CSI processes for CSI reporting, the indication received from the base station is an indication to disable inter-subframe channel interpolation of CSI-RS estimates across subframes for a particular CSI process of the wireless device, and receiving the indication comprises receiving the indication comprised in an RRC information element that configures the particular CSI process of the wireless device.
 21. The method of claim 15 further comprising: receiving a configuration of a set of CSI-RS resources for the wireless device.
 22. The method of claim 21 wherein the CSI report is for a subset of the set of CSI-RS resources configured for the wireless device.
 23. The method of claim 21 wherein receiving the configuration of the set of CSI-RS resources comprises receiving the configuration of the set of CSI-RS resources from the base station via Radio Resource Control, RRC, signaling.
 24. The method of claim 21 wherein the configuration of the set of CSI-RS resources is semi-static.
 25. The method of claim 21 wherein the set of CSI-RS resources is specific to a CSI process of the wireless device.
 26. The method of claim 21 wherein the base station transmits beamformed CSI-RS, and beams used on the set of CSI-RS resources configured for the wireless device are dynamically changed.
 27. A wireless device in a cellular communications network to provide Channel State Information, CSI, reporting, comprising: means for receiving an indication from a base station of the cellular communications network to disable inter-subframe channel interpolation of Channel State Information Reference Symbol, CSI-RS, estimates across subframes; means for receiving an indication from the base station of the cellular communications network to disable combining of CSI Interference Measurement, CSI-IM, estimates across subframes; means for performing one or more CSI-RS measurements with inter-subframe channel interpolation of CSI-RS estimates across subframes disabled in response to receiving the indication; and means for transmitting a CSI report to the base station determined from the one or more CSI-RS measurements.
 28. The wireless device of claim 27 wherein the base station transmits beamformed CSI-RS and reuses the same CSI-RS resources for different beams over time.
 29. A wireless device in a cellular communications network to provide Channel State Information, CSI, reporting, comprising: at least one transmitter; at least one receiver; at least one processor; and memory storing software instructions executable by the at least one processor whereby the wireless device is operative to: receive, via the at least one receiver, an indication from a base station of the cellular communications network to disable inter-subframe channel interpolation of Channel State Information Reference Symbol, CSI-RS, estimates across subframes; receive, via the at least one receiver, an indication from a base station of the cellular communications network to disable combining of Channel State Information Interference Measurement, CSI-IM, estimates across subframes; in response, perform one or more CSI-RS measurements with inter-subframe channel interpolation of CSI-RS estimates across subframes disabled; and transmitting a CSI report to the base station determined from the one or more CSI-RS measurements. 